1. Field of the Invention
The present invention relates to a sheet nipping mechanism applicable to an image forming apparatus, such as electrophotographic copiers, laser printers or facsimile machines. The present invention also relates to an image forming apparatus provided with the sheet nipping mechanism.
2. Description of the Related Art
There has heretofore been known a sheet nipping mechanism applied to an image forming apparatus as represented by electrophotographic copiers, laser printers or facsimile machines. This nipping mechanism is provided in a fixing device disposed downstream of a transfer belt for transporting a sheet having a toner image transferred thereonto through a photosensitive drum. The nipping mechanism comprises a fixing roller having a heater embedded therein, and a pressure roller disposed in opposed relation to the fixing roller while allowing the lower portion thereof to be in close contact with the fixing roller. The fixing roller is formed in a so-called reverse-crown (bow-tie-like, concave or reverse-tapered) shape whose diameter is gradually reduced in a direction extending from each of the opposite transverse ends to the central region of the fixing roller.
Thus, when a sheet is sandwiched between the fixing and pressure rollers, both transverse edges of the sheet are nipped more strongly than its central region to thereby receive forces oriented outward in opposite directions, respectively. The resulting tensile force prevents the sheet from being crimpled.
Such a nipping mechanism is disclosed, for example, in Japanese Parent Laid-Open Publication Nos. 9-114292 and 2000-293060. Specifically, the Japanese Parent Laid-Open Publication No. 9-114292 discloses a nipping mechanism in which each of fixing and pressure rollers is formed in a reverse-crown shape designed such that the amount of reverse crown in each of the rollers is set at 100 μm or more, and the total amount of reverse crown in the rollers is set in the range of 250 to 500 μm. Further, a guide plate to be disposed between a transfer belt and a nip zone is designed such that the height position of the guide plate is located below a line extending in a direction of the tangent to the nip zone between the fixing and pressure rollers. The publication describes that the nipping mechanism employing the above measure could drastically reduce crimples otherwise occurring on a sheet.
Further, the Japanese Parent Laid-Open Publication No. 2000-293060 discloses a nipping mechanism in which an embossed protrusion is formed in the transversely central region of a guide plate to be disposed between a transfer belt and a nip zone. The shape of the embossed protrusion is designed to guide a sheet being transported such that the trailing edge of the sheet is lowered as it gets close to the nip zone. The publication describes that the nipping mechanism employing the embossed protrusion can prevent image distortions due to bouncing in the trailing edge of a large-size sheet.
However, under the recent circumstances where the speeding up in the sheet-feeding and sheet-discharge to/from an image forming apparatus are driven forward, such a high-speed image forming apparatus is essentially required to have more enhanced nipping conditions for a sheet in a nip zone defined by fixing and pressure rollers. The conventional nipping mechanisms as disclosed in the Japanese Parent Laid-Open Publication Nos. 9-114292 and 2000-293060 cannot reliably prevent crimples occurring on a sheet in a nip zone and distortions in a transferred image due to the bouncing phenomenon, and it has been strongly expected to provide an improved nipping mechanism.